This invention relates to a variable displacement axial piston pump in which a thrust plate is mounted on a rocker cam which is received in a rocker cradle and is pivoted in the cradle by a stroking piston to change the displacement of the pump. More particularly, this invention relates to a means for connecting the stroking piston to the rocker cam.
In a variable displacement axial piston pump in which a thrust plate is mounted on a rocker cam, the rocker cam is pivoted in a rocker cradle to change the displacement of the pump. Typically, the rocker cam rotates about an axis normal to the axis of rotation of the pump cylinder barrel. A fluid motor is commonly used to drive the rocker cam. One type of fluid motor is a hydraulic stroking piston and cylinder. The stroking piston is connected to the rocker cam such that linear displacement of the piston results in rotary displacement of the rocker cam.
One means of connecting a hydraulic stroking piston to a rocker cam is shown in U.S. Pat. No. 3,803,987 which is assigned to the assignee of the instant invention. In that device a control arm is rigidly affixed to the rocker cam and projects rearwardly beyond the curved cam surface which is partially received in a rocker cradle. The distal end of the control arm is bifurcated and receives a pin mounted in the central portion of a stroking piston. Linear motion of the stroking piston causes the cam to rotate in the rocker cradle. A problem with this type of mechanical connection between the stroking piston and the rocker cam is that relative movement between the rocker cam and stroking piston can occur because of the tolerances and clearances which are necessary to enable assembly of the parts and to enable the parts to move relative to each other. A particular problem with a piston pump is that when the unit is operating the center of pressure moves. This causes the load on parts such as the pin and arm to change and results in wear of this joint. As the amount of wear increases, the amount of backlash or lost motion between the pin and arm increases. This makes precise positioning of the rocker cam by the stroking piston difficult.
It is possible to eliminate some of the backlash in the type of structure described above by having a spring-loaded piston bear against one side of the control arm and a hydraulically operated stroking piston act against the other side of the control arm. A problem with this structure is that wear still occurs at the interface of the piston and the control arm.
It is desirable to provide a variable displacement, rocker cam type, axial piston pump with a connection between a stroking piston and the rocker cam which has no lost motion and in which wear of the parts at the interface of the cam and the stroking piston is eliminated.
It is also desirable to provide a variable displacement, rocker cam type, axial piston pump with a connection between the stroking piston and the rocker cam which is easy to assemble and relatively inexpensive.